1. Field of the Invention
This invention relates to solid state lighting and in particular to solid state lighting utilizing near field mixing of light from discrete emitters.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
In order to use an LED chip in a circuit or other like arrangement, it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light focusing and the like. An LED package also includes electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. In a typical LED package 10 illustrated in FIG. 1a, a single LED chip 12 is mounted on a reflective cup 13 by means of a solder bond or conductive epoxy. One or more wire bonds 11 connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. The reflective cup may be filled with an encapsulant material 16 which may contain a wavelength conversion material such as a phosphor. Light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly is then encapsulated in a clear protective resin 14, which may be molded in the shape of a lens to collimate the light emitted from the LED chip 12. While the reflective cup 13 may direct light in an upward direction, optical losses may occur when the light is reflected (i.e. some light may be absorbed by the reflector cup due to the less than 100% reflectivity of practical reflector surfaces). In addition, heat retention may be an issue for a package such as the package 10 shown in FIG. 1a, since it may be difficult to extract heat through the leads 15A, 15B.
A conventional LED package 20 illustrated in FIG. 1b may be more suited for high power operations which may generate more heat. In the LED package 20, one or more LED chips 22 are mounted onto a carrier such as a printed circuit board (PCB) carrier, substrate or submount 23. A metal reflector 24 mounted on the submount 23 surrounds the LED chip(s) 22 and reflects light emitted by the LED chips 22 away from the package 20. The reflector 24 also provides mechanical protection to the LED chips 22. One or more wirebond connections 11 are made between ohmic contacts on the LED chips 22 and electrical traces 25A, 25B on the submount 23. The mounted LED chips 22 are then covered with an encapsulant 26, which may provide environmental and mechanical protection to the chips while also acting as a lens. The metal reflector 24 is typically attached to the carrier by means of a solder or epoxy bond.
Techniques for generating white light from a plurality of discrete light sources have been developed that utilize different hues from different discrete light sources, such as those described in U.S. Pat. No. 7,213,940, entitled “Lighting Device and Lighting Method”. These techniques mix the light from the discrete sources to provide white light. In some applications, mixing of light occurs in the far field such that when viewed directly the different hued sources of light can be separately identified, but in the far field the light combines to produce light which is perceived as white. One difficulty with mixing in the far field is that the individual discrete sources can be perceived when the lamp or luminaire is viewed directly. Accordingly, the use of only far field mixing may be most appropriate for these lighting applications where the light sources are mechanically obscured from a user's view. However, mechanically obscuring the light sources may result in lower efficiency as light is typically lost by the mechanical shielding.
Different lamp or luminaries have been developed to more efficiently mix light from the discrete sources to minimize their visibility. The LR6 lamp, commercially available from Cree, Inc. (www.creelighting.com) utilizes a “mixing chamber” where light is reflected in a cavity between a lens and the light sources and passes through a diffuser which obscures the individual sources. Thus, the LR6 lamp appears to have a single light source in much the same way as an incandescent lamp appears to have a single source, even though the LR6 lamp utilizes multiple discrete sources.
While the mixing chamber approach has resulted in very high efficacies for the LR6 lamp of approximately 60 lumens/watt, one drawback of this approach is that a minimum spacing is required between the diffuser lens (which can be a lens and diffuser film) and the light sources. The actual spacing can depend on the degree of diffusion of the lens but, typically, higher diffusion lenses have higher losses that lower diffusion lenses. Thus, the level of diffusion/obscuration and mixing distance are typically adjusted based on the application to provide a light fixture of appropriate depth. In different lamps, the diffuser can be 2 to 3 inches from the discrete light sources, and if the diffuser is closer the light from the light sources may not mix sufficiently. Accordingly, it can be difficult to provide very low profile light fixtures utilizing the mixing chamber approach.
Current LED packages (e.g. XLamp® LEDs provided by Cree, Inc.) can be limited in the level of input power and for some the range is 0.5 to 4 Watts. Many of these conventional LED packages incorporate one LED chip and higher light output is achieved at the assembly level by mounting several of these LED packages onto a single circuit board. FIG. 2 shows a sectional view of one such distributed integrated LED array 30 comprising a plurality of LED packages 32 mounted to a substrate or submount 34 to achieve higher luminous flux. Typical arrays include many LED packages, with FIG. 2 only showing two for ease of understanding. Alternatively, higher flux components have been provided by utilizing arrays of cavities, with a single LED chip mounted in each of the cavities. (e.g. TitanTurbo™ LED Light Engines provided by Lamina, Inc.).
These LED array solutions are less compact than desired as they provide for extended non-light emitting “dead space” between adjacent LED packages and cavities. This dead space provides for larger devices, and can limit the ability to diffuse light from the LED packages and can limit the ability to shape the output beam by a single compact optical element like a collimating lens or reflector into a particular angular distribution. This makes the construction of solid state lighting luminaries that provide for directed or collimated light output within the form factor of existing lamps or even smaller difficult to provide. These present challenges in providing a compact LED lamp structure incorporating an LED component that delivers light flux levels in the 1000 lumen and higher range from a small optical source.